The present invention involves a nanostructure, coating surface for improved adhesion to materials. More specifically, the invention is a laminate product produced by applying a coating having a roughened surface onto a substrate, and the method of producing this product.
In the thin film industry, it is often desirable to have a thin film attached or coated onto a thicker, substrate of the same or different material. To increase adhesion to the substrate, the surface of the thin film contacting the substrate may be roughened by etching or other processes. These processes are difficult to control and can reduce the integrity of the thin film itself, unless the film is relatively thick. In addition, the etching processes involved use environmentally unsafe, waste materials that must be cleaned, recycled and/or disposed of. One such application of thin films involves producing a copper thin film by deposition copper onto a first temporary substrate, and then transferring the thin film onto a final substrate for the production of integrated circuits, printed circuit boards and other electronic applications. In order to apply the thin film to the final product, the thin film must be capable of being easily peeled away from the temporary substrate, and at the same time must adhere to the temporary substrate well enough to remain in place during handling. The temporary substrate (often aluminum or copper) is then peeled off of the copper thin film, leaving the thin film of copper on the final substrate. While there are prior art methods of forming these thin films on temporary substrates, some of these methods require a vacuum environment, which prohibits the use of some materials and makes cooling of the substrate difficult. The present invention overcomes these disadvantages and others by using combustion chemical vapor deposition CCVD or concentrated heat deposition CHD to directly coat the thin film onto the temporary substrate. This results in a thin film that is firmly supported by a substrate for handling purposes, yet easily peeled from this substrate for use. In addition, the product produced using the disclosed methods produces a roughened exposed surface having nanostructure features that interact with the final substrate, thereby producing a stronger adhesion between the thin film and the final permanent substrate and product, while providing a desired thin continuous layer.
U.S. Pat. No. 3,969,199, issued on Jul. 13, 1976 to Berdan et al., discloses a method of coating aluminum with a strippable copper deposit. This method involves pre-treating the aluminum carrier with an alkaline, aqueous, alkali metal zincate solution containing a minor amount of water-soluble salt. The salt is selected from iron, cobalt and nickel. This temporary coating is then removed using acid. By pre-treating the aluminum carrier in this manner, the initial copper electroplated to the aluminum consists of a very high density of small copper nuclei. This results in peel strengths not greater than 2 lbs. per inch width. While the pretreating methods disclosed in this patent may be useful with the present invention, there is no discussion concerning the roughening of the exposed copper surface.
Metal-clad laminates are the subject of U.S. Pat. No. 3,984,598, issued on Oct. 5, 1976 to Sarazin et al. These laminates comprise a metal coating about 1 to 20 microns thick that is deposited on a substrate, after treating the substrate with a release agent. One example given is coating stainless steel with a copper coating after treating the stainless steel with a silane release agent. The upper side of the copper is treated by passing a high currant density and oxidizing the surface using heat. The oxidized surface is treated with a silane bonding agent and is then bonded to a glass epoxy resin laminate. The stainless steel is then removed. While a high degree of adhesion between the copper coating and the glass epoxy resin laminate is achieved using this method, a number of steps are involved, resulting in a costly process. In contradistinction, the present invention roughens the exposed copper (or other material) surface during the coating operation, thus reducing costs as well as the effect on the environment. Furthermore, the larger features associated with the oxidized surface of the copper reduces the overall conductivity per unit weight of the copper, as opposed to the product of the present invention that simply roughens a pure copper surface with smaller features, enabling thinner films, thereby requiring less copper and thus faster etching times
In U.S. Pat. No. 4,357,395, issued on Nov. 2, 1982., U.S. Pat. No. 4,383,003, issued on May 10, 1983 and U.S. Pat. No. 4,431,710, issued on Feb. 14, 1984, all to Lifshin et al., a number of transfer lamination methods and products are disclosed. The most pertinent of these methods and products is illustrated in FIG. 6 of the ""395 Patent. An aluminum carrier sheet is first treated with a release agent (such as silicone dioxide, silicon oxide or soda-lime window glass). A copper coating is then applied by sputtering or other coating technique resulting in a thin film (up to 25 microns) copper layer having a relatively small grain size. The exposed surface of the copper coating is then treated electrolytically or by other methods to alter the morphology of the copper surface. This increases the mechanical interlocking of the copper when bonded to another surface. One such method involves treating the copper surface in a baths of progressively weaker concentrations of copper sulfate. The details (grain or relief sizes) of the roughened copper surface are not disclosed, however, peel strengths on the order of 8 pounds per inch are achieved. As with other known methods, the methods discussed in these patents involve many steps to produce the final product. In addition, while the final product does include a roughened copper surface, the features of the surface are non-uniform and larger when compared to the nanostructure surface of the present invention. This can result in areas having greater adhesion than other areas, as well as areas with varying current carrying capabilities. By providing a surface with nanostructure features, the present invention provides uniform adhesion across the entire surface using a minimum of additional copper or other coating material.
U.S. Pat. No. 5,057,372, issued on Oct. 15, 1991 to Imfeld et al., is directed to a multi-layer film and laminate for use in producing printed circuit boards. The multi-layer film acts as a protective carrier sheet for a metal foil such as copper. An adhesive layer is provided on the surface of the carrier sheet. The adhesive layer is heated or softened to create a releasable bond between the copper foil and the carrier sheet. After the film/foil laminate is placed in a heated press for lamination or molding to the prepreg, the carrier sheet is easily removed. Peel test between the film and foil are between 0.4 pounds/in-width and 0.005 pounds/in-width and preferably between 0.1 pounds/in-width and 0.01 pounds/in-width. This patent is directed mainly to the interface between the film and the foil, and therefore, details concerning the exposed copper surface, or the copper foil production method used, are not disclosed.
An easily peelable or chemically strippable laminate is described in U.S. Pat. No. 5,332,975, issued on Jun. 21, 1994 to Nagy et al. The laminate includes an aluminum layer with an aluminum oxide layer. A thin layer of copper foil is then electroplated on the aluminum oxide, and a thin layer of brass is electroplated on the copper. This results in a copper deposit, which exhibits a low porosity, while the brass layer provides a thermal barrier between the polymeric substrate and the copper foil. The aluminum oxide layer acts as a release agent for the aluminum carrier. The peel strength between the copper and aluminum layers is dependent on the thickness of the aluminum oxide layer and preferably ranges between 0.1 and 0.5 lb./in. While the brass layer is cited as minimizing peel strength degradation between the copper layer and the polymeric substrate, there is no discussion of surface roughening of the copper surface.
None of the above references and patents, taken either singly or in combination, is seen to describes the instant invention as claimed.
The present invention is directed to a thin film conductive product having enhanced adhesive properties, as well as a laminate product with this thin film product embedded therein or thereon. As described above, one use of these thin films is in the circuit board industry wherein the product is a thin film of conductive material such as copper that is first deposited onto a temporary substrate of aluminum (or alternatively, copper). In a further embodiment, the product of the present invention is the thin film is overlain with prepreg or other dielectric circuit board material, with the temporary substrate removed. This dielectric material can have the thin film conductor on one side or both sides for use by circuit board or other manufacturers. In yet a further embodiment, the product of the present invention is a laminate product that includes the thin film attached to dielectric circuit board material with an additional conductive material coated on the thin film. Etching or any known, circuit making method can then be used to create discrete conductor lines or areas to produce a final product as described below. It should be As noted that the present invention is useful with a large number of different materials and applications. The examples described below involve coating a thin film of copper onto a temporary aluminum substrate, as is often used in circuit board production, however, these are simply examples and are not intended to be limiting. The basic objective is to produce a high level of adhesion (greater than 4 lbs./in-width and preferably greater than 6 lbs./in-width) between the conductor and the dielectric insulator (normally epoxy based), while producing a relatively low peel strength (less than 2 lbs./in-width) between the copper foil and the temporary aluminum substrate. Of course, the peel strength between the thin film conductor and aluminum should be high enough (greater than 0.05 lbs./in-width), such that the two do not separate during handling.
To achieve the above objectives, the examples of the present invention use a concentrated heat deposition CHD technique that produces a copper thin film with very low porosity and smooth surface adjacent the aluminum substrate. At the same time, this technique produces an inherent roughening and high porosity of the exposed surface of the copper. This roughened surface is not the typical surface produced in prior art methods such as oxidation or etching, which result in substantially thicker and thinner areas of the foil with numerous features greater than one micron across. In contradistinction, the deposition method used to produce the thin film of the instant invention results in a surface containing a somewhat uniform distribution of mostly nanostructures. The term xe2x80x9cnanostructuresxe2x80x9d is intended to refer to surface features with diameters or heights in the sub-micron range. These nanostructures produce a uniform adhesion, while reducing the amount of material used to assist in the adhesion between the foil and the final substrate. In addition, once removed from the temporary substrate, the resulting thin film has a very smooth upper surface that closely conforms to the surface of the temporary substrate on which it was deposited. When chemically roughening the surface, a thicker copper film is needed to help minimize pinholes being formed by over-treatment. With the present invention, continuous base coatings as thin as 10-1000 nm can be grown with surface structures several times larger attached thereto. Chemical processing yields surface structures nearly the same or less in height then the dense layer base.
Many other deposition techniques may be used to produce the thin film of the present invention, depending on the materials involved. One such technique is combustion chemical vapor deposition CCVD, as described in applicant""s own U.S. Pat. Nos. 5,652,021, 5,858,465 and 5,863,604, all of which are hereby incorporated by reference. Some materials (copper in particular), however, are more difficult to deposit using CCVD, as a low oxygen environment is required. For the deposition of these materials, a non-combustion energy source can be provided. These heat sources can be hot gasses, heated tubes, radiant energy, microwave and energized photons (such as infrared or laser) to name a few. Further details of a suitable deposition technique are disclosed in co-pending U.S. patent application Ser. No. 09/067,975 entitled xe2x80x9cAPPARATUS AND PROCESS FOR CONTROLLED ATMOSPHERE CHEMICAL VAPOR DEPOSITIONxe2x80x9d, and hereby incorporated by reference. The examples provided below in the detailed description section were produced using hot gasses as the energy source. A precursor solution containing copper is atomized, by passing the solution through a small diameter tube. This atomization technique is more fully described in co-pending U.S. patent application Ser. No. 08/691,853, now U.S. Pat. No. 5,997,956, entitled xe2x80x9cCHEMICAL VAPOR DEPOSITION AND POWDER FORMATION USING THERMAL SPRAY WITH NEAR SUPERCRITICAL AND SUPERCRITICAL FLUID SOLUTIONSxe2x80x9d and hereby incorporated by reference. A second tube surrounds the small tube and hot gasses are fed into the larger tube to provide the energy required for the atomization. The larger tube truncates in an extended collar that conforms to and is essentially parallel with the substrate to be coated. As the hot gasses exit the large tube they are traveling at a high rate of speed (50-100 feet/minute and greater). The collar routes the hot gasses in a radial direction thereby forming a barrier zone that prevents contamination (such as oxidation) by blocking atmospheric gasses from entering the deposition zone.
Accordingly, it is a first object of the invention to produce a thin film having a structured surface for increased adhesion to a substrate.
It is another object of the invention to provide a laminate including a thin film on a temporary substrate wherein the thin film has an exposed, structured surface providing greater adhesion to a final substrate than the adhesion between the thin film and the temporary substrate.
It is still another object of the invention to produce a thin film on a substrate in an open atmosphere environment, without degradation of the thin film by atmospheric gasses.
It is yet a further object of the invention to produce a thin film having a dense thickness less than 200 nm that also exhibits a high degree of adhesion to a final substrate.
It is still another object of the invention to provide a copper thin film on an aluminum or copper substrate for protection during handling, wherein the thin film can be easily transferred to an insulating substrate for use in producing printed circuit boards, integrated circuits and other electronic products.
These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.